Electric furnaces are required to have various performance characteristics such as an ability to rapidly elevate the temperature, superior stability in a high-temperature oxidizing atmosphere, good durability against continuous use at high temperatures and against repeated increase and decrease of temperatures, a long length of soaking zone in a heating chamber, high handleability and the like.
Conventional electric furnaces equipped with resistance heating elements have the structure shown in FIGS. 1A and 1B wherein a plurality of cylindrical heating elements 12 are arranged around a furnace tube 13 used as a heating chamber, and wherein said heating elements are surrounded with a heat-resistant tube 14, heat-insulating layers 15 and 16 and an external casing 17 in this order.
However, the electric furnaces of this structure are adapted to indirectly heat the heating chamber by radiation of Joule heat from the cylindrical heating elements 12, and thus require a number of heating elements 12, involving a drawback of having a complicated structure and a great size as compared with the effective volume of the heating chamber. Such electric furnaces have posed problems such as poor durability due to low heating efficiency. Further, other problems exist in terms of the service life of heating elements and economy, because the constituent materials for electric furnaces have a large heat capacity and great electric power is required for heating to a predetermined temperature and in maintaining the temperature.